Field of the Invention
The present invention relates to an image-fixing member (hereinafter referred to as a “fixation member”) which is particularly employed in a fixation unit of an image-forming apparatus such as an electrophotographic printer or copying machine.
Background Art
Generally, an image-fixation unit of an image-forming apparatus such as an electrophotographic printer or copying machine employs a fixation belt formed of an electrocast layer (nickel (Ni), Ni/copper (Cu)/Ni, PI (polyimide resin), SUS (stainless steel), etc.), an elastic layer, a release layer (fluororesin tube), etc.; a fixation roller or a pressure roller formed of a metallic core, an elastic layer, a release layer, etc.; or the like.
In a typical fixation apparatus 100 shown in FIG. 1, a fixation belt 104 (a fixation member in the form of a belt) moves with a pressure roller 101 in rotation, and a fixation roller (an inner roller 105) which is heated by means of heating means 103 disposed in a space 102 rotates with the pressure roller 101, whereby an unfixed toner image 107 on a recording medium 106 (e.g., a paper sheet) is fixed through heat and pressure during passage of the recording medium between the fixation belt 104 and the pressure roller 101. In the fixation apparatus 100, the fixation belt 104 is disposed so as to face oppositely the toner-provided surface of the recording medium 106. Thus, the fixation belt 104 comes into direct contact with the unfixed toner image 107.
In the above fixation apparatus 100, the heating means 103 is disposed in the space defined by the fixation belt 104 by the mediation of a heating roller 108. Other than this configuration of the fixation apparatus, there exist, for example, a fixation apparatus in which heating means is disposed so as to face oppositely the outside of the fixation belt, and a fixation apparatus having a roller-shape fixation member. Also, a variety of heating means are employed in such fixation members. Examples thereof include a halogen lamp; resistance heaters such as a metallic resistor, a ceramic heater, and a carbon heater; an electromagnetic induction heater (IH); and a microwave heater.
Meanwhile, a problem of toner off set is involved in conventional fixation apparatuses. “Toner offset” refers to toner remaining on a fixation member during passage of a recording paper sheet and includes offset caused by an electrical reason (e.g., electrostatic offset) and offset caused by a thermal reason (e.g., hot offset or cold offset). Thus, toner image fixation must be carried out under such conditions that occurrence of toner offset is prevented.
In a recent trend for obtaining a toner image of higher quality, the surface of a fixation member which comes into direct contact with toner is required to have high releasability with respect to toner or paper dust (hereinafter may be refereed to simply as “releasability”). Thus, the amount of such dust must be reduced by lowering the surface energy. In order to reduce the surface energy of a fixation member, there have been proposed fixation members which employ, at least on a surface thereof, a fluororesin such as PFA (perfluoroalkoxyfluororesin) or PTFE (polytetrafluoroethylene) (see, for example, Patent Documents 1 and 2).
Notably, the polarity of a toner employed in an image-forming apparatus varies depending on the type of the apparatus. When a positively charged toner is used, electrostatic offset must be prevented. In one disclosed technique, graphite powder is charged into a heat-resistant offset-preventing coating layer formed of fluororesin which has been disposed on the surface of a heat/fixation roller serving as a fixation member, whereby electrostatic offset of the fixation member is prevented (see, for example, Patent Document 3).
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2014-112201
Patent Document 2: Japanese Patent No. 4790002
Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 1997-237007
According to Patent Document 3, graphite is added to a fluororesin tube forming the release layer of a fixation member, to thereby provide the release layer with electrical conductivity, whereby electrostatic offset of the fixation member can be prevented. However, when the fluororesin tube has electrical conductivity, the surface energy of the tube increases, and releasability decreases, which is problematic. Thus, in provision of a fixation member, there is demand for a fluororesin tube having suitable electrical conductivity and high releasability.